docs: def _get_datetimes(self, timestep=1):
          """List of datetimes based on design day date and timestep."""
          start_moy = DateTime(self._month, self._day_of_month).moy
          if timestep == 1:
              start_moy = start_moy + 30
          num_moys = 24 * timestep
          return tuple(
              DateTime.from_moy(start_moy + (i * (1 / timestep) * 60))
              for i in xrange(num_moys)
          )